1. Field of the Invention
The present invention relates to an induction heating coil used as the heating device of a semiconductor single crystal grower and more particularly to an induction heating coil used for a crystal grower in which a dopant substance is uniformly dispersed in a grown crystal during the growth by FZ method (an abbreviation of the so-called floating zone melting method).
2. Description of the Prior Art
An apparatus used for growing a semiconductor single crystal by FZ method comprises an upper shaft for holding a raw material rod, a lower shaft for holding a seed crystal made of a single crystal semiconductor having a smaller diameter and a heating device arranged such that it causes the raw material rod to fuse by heating.
In growing a single crystal with the apparatus above-mentioned, an end of the raw material rod is fused to be connected to the free tip of the seed crystal to produce a molten zone and in succession a narrow single crystal rod portion is grown dislocation free on the seed crystal and then the raw material rod starts moving downward together with the seed crystal relative to the heating device to drive the molten zone toward the other end of its own along the rod, while being rotated about its axis and thereby almost the whole length is crystallized in the shape of a rod.
An apparatus for growing a crystal rod requires that a raw material is fully fused without any core portion left not fused within a narrow molten zone therein during zone melting. Therefore, as a heating device, which meets the requirement, for example, an induction heating coil having a construction of a single turn and being flat is well known in the prior art.
Such well known induction heating coils include, for example, an induction heating coil 1 as shown in FIG. 2, wherein a hollow conductor 4 is bent in the shape of a ring and both of the ends are located face to face with a gap 3 interposed therebetween in the closest relation that does not cause undesirable sparks with each other. The induction heating coil is electrically connected to a pair of power source terminals 5 at the two respective points on the outer vertical surface in the vicinity of both the coil ends and the section in parallel with the radial direction is in the shape of a wedge (the Japanese second publication No.51-24964, hereinafter referred to as the first prior art).
Such a construction of the coil of the first prior art may give rise to a current path almost a perfect geographic symmetry to the coil center in the surface of the coil. When a high-frequency current is supplied through the power source terminals 5 to the coil 1, an electromagnetic field is formed on and around the coil 1 and the field is almost uniform in strength in the area surrounded by the inner periphery 2, within which a raw material rod is vertically disposed therethrough and thereby the rod is partly heated to form a molten zone therein.
However, an induction coil of the first prior art has the end gap 3 being formed in parallel with a plane perpendicular to a tangent at the ends of the peripheries of the coil and thereby uniformity is caused in the vicinity of the gap 3. On the other hand, the high-frequency current supplied from the power source terminals 5 flows on and along the current path 7 depicted in FIG. 2 due to its nature, which takes a shortest path in principle. As a result, an induction coil 1 of this kind comes to give rise to regions 8 of a lower-density current in the surface thereof and thereby again causes another ununiformities thereon and therearound. In the mean time, the regions 8 are only in part shown in FIG. 2 for clarity.
Such an uneven magnetic field brings about an uneven heating capacity distribution over the coil and thereby a growing crystal is adversely affected in quality. For instance, when a raw material rod is rotated and transferred relative to the induction heating coil 1 in such a uneven magnetic field, a pair of a higher and the following lower layer in a dopant concentration is periodically produced in the growing portion within one revolution of the rod due to differentials of local heating capacity caused by the uniformity in the uneven magnetic field surrounding the coil. In one revolution of the rod, a dopant concentration of the growing front of the crystal becomes lower in a portion at a higher temperature and conversely the dopant concentration becomes higher in a portion at a lower temperature, which is called "periodical resistivity variation". Semiconductor wafers produced by slicing a single crystal rod having such a repetition in bulk of the combination of higher and lower concentrations as described above bring about fluctuations in characteristics of a semiconductor device fabricated starting therefrom due to microfluctuation in resistivity observed in the regions of periodical resistivity variations.
A contrivance to compensate the faults observed in the first prior art has been made, which is constructed in such a manner that as shown in FIG. 3, a single-turn flat coil 1 has two slits 6 extending from the inner periphery 2 towards the outer periphery 4 and further to the contrary another three slits 6 extending from the outer periphery in the reverse way (the Japanese first publication No.52-30705, hereinafter referred to as the second prior art). In this second prior art, the induction heating coil 1 has a plurality of slits 6 with all the widths thereof being same in dimension as that of the gap 3 at the ends of the coil 1. The slits 6 are arranged at equal distance intervals in the circumferential direction. The configuration of said slits has a geometrical periodicity about the center of the coil. Under such conditions a high-frequency current is controlled to flow in the surface of the coil 1 so as to keep an axial symmetry to the central axis of the coil.
Even an induction heating coil in the second prior art, however, is not able to avoid occurrence of lower current density regions 8 in the surface of the coil, as shown in FIG. 3. This phenomenon originates in the nature of a high-frequency current flowing a shortest path and in the structural restriction of the induction heating coil itself as well.
In particular, the induction heating coil 1 has to have such a configuration for cooling the coil that a coolant may flow inside of the coil having a hollow structure. In line with the requirement, narrow regions 9 should be provided at least both on the outer periphery side and on the inner periphery side of the coil, where the widths of the narrow regions 9 should be large enough to allow the coolant water to flow through. Consequently, the high-frequency current does not take a path along and in close vicinity to the inner periphery and along both the sides of slits and thereby the lower-density current regions 8 are created in the surface of the coil. With this configuration of the coil 1, the current path 7 is not able to be selected nearer enough toward the inner periphery and thereby a stirring power in the core portion of the molten zone becomes week, so that resistivities in the core portion of a semiconductor crystal rod thus produced are the lower corresponding to the decrease in the stirring power.